The present invention relates to a power transfer mechanism for a four-wheel drive motor vehicle.
A power transfer mechanism for a four-wheel drive automotive vehicle is known. The known mechanism includes a sub-transmission which may be shifted to establish a high-range mode or a low-range mode. The mechanism also includes a center differential which absorbs a difference in rotation between two output members for front axle and rear axle of an associated four-wheel drive vehicle. The mechanism, therefore, requires a large number of component parts, resulting in a construction which is bulky, heavy and expensive to manufacture.
In order to alleviate the above deficiencies, there has been proposed a power transfer mechanism incorporating a single planetary gear assembly as disclosed in U.S. Pat. No. 4,074,591. The planetary gear assembly includes a shifting mechanism. The shifting mechanism may be shifted to establish a high-range mode wherein the carrier acts as an input member and the sun and ring gears as output members to establish torque proportioning differential action. The mechanism may be shifted to establish a low-range mode wherein the sun gear acts as an input member, the ring gear as a reaction member, and the carrier as an output member to provide locked-up reduction drive. The use of a single planetary gear assembly provides a construction which is less bulky, compact, and less heavy. The shifting mechanism includes a pair of shifting sleeves, each supporting a floating collar. In the high-range mode, one sleeve engages the input with the carrier, and its associated floating collar engages the sun gear with one output member. The other sleeve engages the ring gear with the other output member, and its associated floating collar is disengaged. In the low-range mode, the one sleeve engages the input with the sun gear, and its associated floating collar engages the carrier with one output member. The other sleeve engages the carrier with the other output member, and its associated floating collar grounds the ring gear to the housing. The shifting mechanism is mounted in the housing and disposed radially outwardly of the input and output members and radially inwardly of the sun gear. As a result, this disposition of the shifting mechanism is a stumbling block in reducing the over all dimension of the entire mechanism and causes a complicated structure. The fact that the radial dimension is relatively large poses a problem in its installation in an associated four-wheel drive vehicle because the vehicle floor has to be elevated so as to secure the road clearance high enough for the associated four-wheel drive to take full advantage of the power transfer mechanism, thus sacrificing the vehicle's cabin space, or the road clearance has to be sacrificed so as to maintain the vehicle floor as low as possible and to secure sufficiently large vehicle's cabin, thus preventing the associated four-wheel drive vehicle from exibiting good running performance in passing through the rough terrain. Besides, in the high-range mode, since the input member is engaged with the carrier, and the ring gear is engaged with the output members for transferring torque to the rear axle of an associated four-wheel drive vehicle and the sun gear is engaged with the other output member for transferring torque to the front axle of the four-wheel drive vehicle, torque transferred to the rear axle is always larger than torque transferred to the front axle by a considerable amount owing to considerable difference in length between the arms with which the ring and sun gears turn the associated output members. This causes an associated four-wheel drive to show oversteering characteristic when making a turn. Lastly, since the input is engaged with the sun gear (in the low-range mode) or the carrier (in the high-range mode), the reduction ratio having a sufficiently small value cannot be obtained.
There is a need to provide a power transfer mechanism which establishes at least approximately even distribution in torque between two output members when the mechanism is shifted to the torque proportioning differential drive range.
An object of the present invention is to provide a power transfer mechanism which meets the above mentioned need.